Transcritical refrigerant vapor compression system with charge management

ABSTRACT

A refrigerant vapor compression system includes a refrigerant-to-refrigerant heat exchanger economizer and a flash tank disposed in series refrigerant flow relationship in the refrigerant circuit intermediate a refrigerant heat rejection heat exchanger and a refrigerant heat absorption heat exchanger. A primary expansion valve is interdisposed in the refrigerant circuit upstream of the refrigerant heat absorption heat exchanger and a secondary expansion valve is interdisposed in the refrigerant circuit upstream of the flash tank. The flash tank functions as a refrigerant charge storage reservoir wherein refrigerant expanded from a supercritical pressure to subcritical pressure separates into liquid and vapor phases. A refrigerant vapor bypass line is provided to return refrigerant vapor from the flash tank to the refrigerant circuit downstream of the refrigerant heat absorption heat exchanger. The primary expansion valve and a flow control valve interdisposed in the refrigerant vapor bypass provide refrigerant charge management.

FIELD OF THE INVENTION

This invention relates generally to refrigerant vapor compressionsystems and, more particularly, to refrigerant charge management in arefrigerant vapor compression system operating in a transcritical cycle.

BACKGROUND OF THE INVENTION

Refrigerant vapor compression systems are well known in the art andcommonly used for conditioning air to be supplied to a climatecontrolled comfort zone within a residence, office building, hospital,school, restaurant or other facility. Refrigerant vapor compressionsystems are also commonly used in refrigerating air supplied to displaycases, merchandisers, freezer cabinets, cold rooms or otherperishable/frozen product storage area in commercial establishments.

Refrigerant vapor compression systems are also commonly used intransport refrigeration systems for refrigerating air supplied to atemperature controlled cargo space of a truck, trailer, container or thelike for transporting perishable/frozen items by truck, rail, ship orintermodally. Refrigerant vapor compression systems used in connectionwith transport refrigeration systems are generally subject to morestringent operating conditions due to the wide range of operating loadconditions and the wide range of outdoor ambient conditions over whichthe refrigerant vapor compression system must operate to maintainproduct within the cargo space at a desired temperature. The desiredtemperature at which the cargo needs to be controlled can also vary overa wide range depending on the nature of cargo to be preserved. Therefrigerant vapor compression system must not only have sufficientcapacity and refrigerant charge to rapidly pull down the temperature ofproduct loaded into the cargo space at ambient temperature, but alsooperate efficiently at low load with excess refrigerant charge whenmaintaining a stable product temperature during transport. Additionally,transport refrigerant vapor compression systems are subject to vibrationand movements not experienced by stationary refrigerant vaporcompression systems. Thus, the use of a conventional refrigerantaccumulator in the suction line upstream of the compressor suction inletto store excess refrigerant liquid would be subject to sloshing duringmovement that could result in refrigerant liquid being undesirablycarried through the suction line into the compressor via the suctioninlet thereto.

Traditionally, most of these refrigerant vapor compression systemsoperate at subcritical refrigerant pressures and typically include acompressor, a condenser, and an evaporator, and expansion device,commonly an expansion valve, disposed upstream, with respect torefrigerant flow, of the evaporator and downstream of the condenser.These basic refrigerant system components are interconnected byrefrigerant lines in a closed refrigerant circuit, arranged in accordwith known refrigerant vapor compression cycles, and operated in thesubcritical pressure range for the particular refrigerant in use.Refrigerant vapor compression systems operating in the subcritical rangeare commonly charged with fluorocarbon refrigerants such as, but notlimited to, hydrochlorofluorocarbons (HCFCs), such as R22, and morecommonly hydrofluorocarbons (HFCs), such as R134a, R410A, R404A andR407C.

In today's market, greater interest is being shown in “natural”refrigerants, such as carbon dioxide, for use in air conditioning andtransport refrigeration systems instead of HFC refrigerants. However,because carbon dioxide has a low critical temperature, most refrigerantvapor compression systems charged with carbon dioxide as the refrigerantare designed for operation in the transcritical pressure regime. Inrefrigerant vapor compression systems operating in a subcritical cycle,both the condenser and the evaporator heat exchangers operate atrefrigerant temperatures and pressures below the refrigerant's criticalpoint. However, in refrigerant vapor compression systems operating in atranscritical cycle, the heat rejection heat exchanger, which is a gascooler rather than a condenser, operates at a refrigerant temperatureand pressure in excess of the refrigerant's critical point, while theevaporator operates at a refrigerant temperature and pressure in thesubcritical range. Thus, for a refrigerant vapor compression systemoperating in a transcritical cycle, the difference between therefrigerant pressure within the gas cooler and refrigerant pressurewithin the evaporator is characteristically substantially greater thanthe difference between the refrigerant pressure within the condenser andthe refrigerant pressure within the evaporator for a refrigerant vaporcompression system operating in a subcritical cycle.

It is also common practice to incorporate an economizer into therefrigerant circuit for increasing the capacity of the refrigerant vaporcompression system. For example, in some systems, arefrigerant-to-refrigerant heat exchanger is incorporated into therefrigerant circuit as an economizer. A first portion of the refrigerantleaving the condenser passes through a first pass of the heat exchangerin heat exchange with a second portion of the refrigerant passingthrough the second pass of the heat exchanger. The second portion of therefrigerant typically constitutes a portion of the refrigerant leavingthe condenser that is diverted through an expansion device wherein thisportion of the refrigerant is expanded to a lower pressure and a lowertemperature vapor or vapor/liquid mixture refrigerant before this secondportion of refrigerant is passed through the second pass of theeconomizer refrigerant-to-refrigerant heat exchanger. Having traversedthe second pass of the economizer heat exchanger, the second portion ofthe refrigerant is thence directed into an intermediate pressure changeof the compression process. The refrigerant in the primary refrigerantcircuit passes through the first pass of the refrigerant-to-refrigeranteconomizer heat exchanger and is thus further cooled before it traversesthe system's main expansion device prior to entering the evaporator.U.S. Pat. No. 6,058,729 discloses a subcritical refrigerant vaporcompression system for a transport refrigeration unit incorporating arefrigerant-to-refrigerant heat exchanger into the refrigerant circuitas an economizer. U.S. Pat. No. 6,694,750 discloses a subcriticalrefrigeration system that includes a first refrigerant-to-refrigerantheat exchanger economizer and a second refrigerant-to-refrigerant heatexchanger economizer disposed in series in the refrigerant circuitbetween the condenser and the evaporator.

In some systems, a flash tank economizer is incorporated into therefrigerant circuit between the condenser and the evaporator. In suchcase, the refrigerant leaving the condenser is expanded through anexpansion device, such as a thermostatic expansion valve or anelectronic expansion valve, prior to entering the flash tank wherein theexpanded refrigerant separates into a liquid refrigerant component and avapor refrigerant component. The vapor component of the refrigerant isthence directed from the flash tank into an intermediate pressure stageof the compression process. The liquid component of the refrigerant isdirected from the flash tank through the system's main expansion valveprior to entering the evaporator. U.S. Pat. No. 5,174,123 discloses asubcritical vapor compression system incorporating a flash tankeconomizer in the refrigerant circuit between the condenser and theevaporator. U.S. Pat. No. 6,385,980 discloses a transcriticalrefrigerant vapor compression system incorporating a flash tankeconomizer in the refrigerant circuit between the gas cooler and theevaporator.

SUMMARY OF THE INVENTION

A transcritical refrigerant vapor compression system having improvedrefrigerant charge management includes a compression device, arefrigerant heat rejection heat exchanger, a refrigerant heat absorptionheat exchanger, and a refrigerant-to-refrigerant heat exchangereconomizer and a flash tank disposed in a primary refrigerant circuit inseries refrigerant flow relationship intermediate a refrigerant heatrejection heat exchanger and a refrigerant heat absorption heatexchanger. A primary expansion valve interdisposed in the refrigerantcircuit in operative association with and upstream of the refrigerantheat absorption heat exchanger and a secondary expansion valveinterdisposed in the refrigerant circuit in operative association andupstream of the flash tank. A refrigerant vapor bypass line establishesrefrigerant vapor flow communication between the flash tank and asuction pressure portion of the primary refrigerant circuit downstreamof the refrigerant heat absorption heat exchanger. A bypass flow controlvalve having an open position and a closed position is interdisposed inthe refrigerant vapor bypass line for controlling the flow ofrefrigerant vapor through the refrigerant vapor bypass line.

The refrigerant-to-refrigerant heat exchanger has a first refrigerantpass disposed in the primary refrigerant circuit downstream of therefrigerant cooling heat exchanger and upstream of the primary expansiondevice and a second bypass disposed in an economizer circuit refrigerantline that extends in refrigerant flow communication from the primaryrefrigerant circuit to an intermediate pressure stage of the compressiondevice. An economizer circuit expansion device is interdisposed in theeconomizer circuit refrigerant line upstream with respect to refrigerantflow of the second refrigerant pass of the refrigerant-to-refrigerantheat exchanger economizer. The economizer circuit expansion device maycomprise an electronic expansion valve or a thermostatic expansionvalve.

In an embodiment, the bypass flow control valve may comprise atwo-position solenoid valve, a pulse width modulated solenoid valve oran electronic expansion valve. In an embodiment, the primary expansionvalve may comprise an electronic expansion valve or a thermostaticexpansion valve. In an embodiment, the secondary expansion valve maycomprise an electronic expansion valve or a fixed orifice expansiondevice.

In an embodiment, the compression device may be single compressor havingat least a first compression stage and a second compression stage. In anembodiment, the compression device may be a first compressor and asecond compressor disposed in the refrigerant circuit in seriesrefrigerant flow relationship with a discharge outlet of the firstcompressor in refrigerant flow communication with a suction inlet of thesecond compressor. In either the single compressor arrangement or thedual compressor arrangement, each compressor may be a scroll compressor,a reciprocating compressor or a screw compressor.

BRIEF DESCRIPTION OF THE DRAWINGS

For a further understanding of the invention, reference will be made tothe following detailed description of the invention which is to be readin connection with the accompanying drawing, where:

FIG. 1 is a schematic diagram illustrating an exemplary embodiment of arefrigerant vapor compression system in accord with the invention;

FIG. 2 is a graph illustrating the pressure to enthalpy relationship forthe exemplary embodiment of the refrigerant vapor compression system ofthe invention illustrated in FIG. 1 operating in a transcritical cycle;

FIG. 3 is a graph illustrating the pressure to enthalpy relationship fora prior art refrigerant vapor compression system operating in atranscritical cycle with a single refrigerant-to-refrigerant heatexchanger economizer; and

FIG. 4 is a graph illustrating the pressure to enthalpy relationship fora prior art refrigerant vapor compression system operating in atranscritical cycle with a single flash tank economizer.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is depicted an exemplary embodiment of atranscritical refrigerant vapor compression system 10 suitable for usein a transport refrigeration system for refrigerating air supplied to atemperature controlled cargo space of a truck, trailer, container or thelike for transporting perishable and frozen goods. The refrigerant vaporcompression system 10 is also suitable for use in conditioning air to besupplied to a climate controlled comfort zone within a residence, officebuilding, hospital, school, restaurant or other facility. Therefrigerant vapor compression system could also be employed inrefrigerating air supplied to display cases, merchandisers, freezercabinets, cold rooms or other perishable and frozen product storageareas in commercial establishments.

The transcritical refrigerant vapor compression system 10 includes amulti-stage compression device 20, a refrigerant heat rejection heatexchanger 40, also referred to herein as a gas cooler, a refrigerantheat absorption heat exchanger 50, also referred to herein as anevaporator, and a primary expansion device 55, such as for example anelectronic expansion valve or a thermostatic expansion valve,operatively associated with the evaporator 50, with various refrigerantlines 2, 4 and 6 connecting the aforementioned components in a primaryrefrigerant circuit.

The compression device 20 functions to compress the refrigerant and tocirculate refrigerant through the primary refrigerant circuit as will bediscussed in further detail hereinafter. The compression device 20 maycomprise a single, multiple-stage refrigerant compressor, for example areciprocating compressor, having a first compression stage 20 a and asecond stage 20 b, or a single compressor, for example a scrollcompressor or a screw compressor, adapted in a conventional manner forinjection of refrigerant, for example via an injection port, into anintermediate pressure point of the compression chamber of thecompressor, whereby the first compression stage 20 a is upstream of theintermediate pressure point and the second compression stage 20 b isdownstream of the intermediate pressure point. The first and secondcompression stages 20 a and 20 b are disposed in series refrigerant flowrelationship with the refrigerant leaving the first compression stagepassing directly to the second compression stage for furthercompression. The compression device 20 may also comprise a pair ofcompressors 20 a and 20 b, connected in series refrigerant flowrelationship in the primary refrigerant circuit via a refrigerant lineconnecting the discharge outlet port of the first compressor 20 a inrefrigerant flow communication with the suction inlet port of the secondcompressor 20 b. The compressors 20 a and 20 b may be scrollcompressors, screw compressors, reciprocating compressors, rotarycompressors or any other type of compressor or a combination of any suchcompressors.

The refrigerant heat rejecting heat exchanger 40 may comprise a finnedtube heat exchanger 42 through which hot, high pressure refrigerantpasses in heat exchange relationship with a cooling medium, mostcommonly ambient air drawn through the heat exchanger 42 by thecondenser fan(s) 44. The finned tube heat exchanger 42 may comprise, forexample, a fin and round tube heat exchange coil or a fin and flatmini-channel tube heat exchanger.

Additionally, the refrigerant vapor compression system 10 of theinvention includes a refrigerant-to-refrigerant heat exchangereconomizer 60 and a flash tank 70 interdisposed in series refrigerantflow relationship in refrigerant line 4 of the primary refrigerantcircuit downstream with respect to refrigerant flow of the gas cooler 40and upstream with respect to refrigerant flow of the evaporator 50. Therefrigerant-to-refrigerant heat exchanger economizer 60 is disposed inrefrigerant line 4 of the primary refrigerant circuit downstream withrespect to refrigerant flow of the gas cooler 40 and upstream withrespect to refrigerant flow of the flash tank 70. Additionally, asecondary expansion device 75, such as for example, an electronicexpansion valve or a fixed orifice device, is interdisposed in theprimary refrigerant circuit intermediate the refrigerant-to-refrigerantheat exchanger economizer 60 and the flash tank 70.

The refrigerant-to-refrigerant heat exchanger economizer 60 includes afirst refrigerant pass 62 and a second refrigerant pass 64 arranged inheat transfer relationship. The first refrigerant pass 62 isinterdisposed in refrigerant line 4 and forms part of the primaryrefrigerant circuit. The second refrigerant pass 64 is interdisposed inrefrigerant line 12 and forms part of an economizer circuit. Theeconomizer circuit refrigerant line 12 connects in refrigerant flowcommunication with an intermediate pressure stage of the compressionprocess. In the exemplary embodiment depicted in FIG. 1, the economizercircuit refrigerant line 12 connect to refrigerant line 4 of the primaryrefrigerant circuit either upstream with respect to refrigerant flow ofthe first pass 62 of the refrigerant-to-refrigerant heat exchangereconomizer 60 and establishes refrigerant flow Alternatively, theeconomizer circuit refrigerant line may connect to refrigerant line 4 ofthe primary circuit downstream with respect to refrigerant flow of thefirst pass 62 of the refrigerant-to-refrigerant heat exchangereconomizer 60. The first refrigerant pass 62 and the second refrigerantpass 64 of the refrigerant-to-refrigerant heat exchanger economizer 60may be arranged in a parallel flow heat exchange relationship or in acounter flow heat exchange relationship, as desired. Therefrigerant-to-refrigerant heat exchanger 60 may be a brazed plate heatexchanger, a tube-in-tube heat exchanger, a tube-on-tube heat exchangeror a shell-and-tube heat exchanger.

An economizer circuit expansion device 65 is disposed in the economizercircuit refrigerant line 12 upstream with respect to refrigerant flow ofthe second pass 64 of the refrigerant-to-refrigerant heat exchangereconomizer 60. The economizer circuit expansion device 65 meters therefrigerant flow that passes through the refrigerant line 12 and thesecond pass 64 of the refrigerant-to-refrigerant heat exchangereconomizer 60 in heat exchange relationship with the refrigerant passingthrough the first pass of the heat exchanger economizer 60 to maintain adesired level of superheat in the refrigerant vapor leaving the secondpass 64 of the heat exchanger economizer 60 to ensure that no liquid ispresent therein. The expansion valve 65 may be an electronic expansionvalve, for example as depicted in FIGS. 1-3, in which case the expansionvalve 65 meters refrigerant flow in response to a signal from acontroller 100 to maintain a desired refrigerant temperature or pressurein refrigerant line 12. The expansion device 65 may also be athermostatic expansion valve, in which case the expansion valve 65meters refrigerant flow in response to a signal indicative of therefrigerant temperature or pressure sensed by the sensing device (notshown) which may be a conventional temperature sensing element, such asa bulb or thermocouple mounted to the refrigerant line 12 downstream ofthe second pass of the heat exchanger economizer 60. The refrigerantvapor passing through the economizer circuit refrigerant line 12 isinjected into the compression device 20 at an intermediate pressurepoint of the compression process. For example, if the compression device20 is a multi-stage reciprocating compressor, refrigerant line 12directs refrigerant vapor directly into an intermediate pressure stageof the reciprocating compressor between the first compression stage 20 aand the second compression stage 20 b. If the compression device 20 is asingle scroll compressor or a single screw compressor, the refrigerantline 12 directs refrigerant vapor into an injection port of thecompression device opening to the compression chamber of the compressiondevice at an intermediate pressure of the compression process. If thecompression device 20 is a pair of compressors 20 a, 20 b, for example apair of scroll compressors, or screw compressors, or reciprocatingcompressors, connected in series, or a single reciprocating compressorhaving a first bank and a second bank of cylinders, the secondeconomizer circuit refrigerant line 12 directs refrigerant vapor into arefrigerant line that connects the discharge outlet port of the firstcompressor 20 a in refrigerant flow communication with the suction inletport of the second compressor 20 b.

The flash tank 70 is interdisposed in refrigerant line 4 of the primaryrefrigerant circuit downstream with respect to refrigerant flow of thefirst pass 62 of the refrigerant-to-refrigerant heat exchangereconomizer 60 and upstream with respect to refrigerant flow of theevaporator 50 to receive the refrigerant flowing through refrigerantline 4. A secondary expansion device 75 is interdisposed in refrigerantline 4 of the primary refrigerant circuit downstream with respect torefrigerant flow of the first refrigerant pass 62 of therefrigerant-to-refrigerant heat exchanger economizer 60 and upstreamwith respect to refrigerant flow of the inlet to the flash tank 70. Highpressure refrigerant vapor passing through refrigerant line 4 isexpanded as it traverses the secondary expansion device 75 to asubcritical pressure and temperature before the refrigerant passes intothe flash tank 70. The secondary expansion device 75 may be anelectronic expansion valve, such as illustrated in FIG. 1, in which casethe secondary expansion valve 75 meters refrigerant flow in response toa signal from a controller 100 to maintain a desired refrigerantpressure in refrigerant line 4 upstream with respect to refrigerant flowof the secondary expansion device 75. The secondary expansion device 75may also simply be a fixed orifice expansion device, in which case therefrigerant pressure in refrigerant line 4 upstream with respect torefrigerant flow of the secondary expansion device 75 will fluctuatedepending upon ambient conditions and the refrigerant flow will beinherently metered in accord with the magnitude of the pressuredifferential across the fixed orifice.

The flash tank 70 defines a separation chamber 72 into which theexpanded refrigerant flows at a subcritical pressure and separates intoa liquid refrigerant portion that collects in the lower portion of theflash tank 70 and into a vapor portion that collects in the upperportion of the flash tank 70 above the liquid level within the flashtank 70. Thus, the flash tank 70 functions as a receiver for storingliquid refrigerant whenever the refrigerant vapor compression system isoperating at a capacity that does not require the system's fullrefrigerant charge.

Additionally, the refrigerant vapor compression system includes arefrigerant line 14 that establishes refrigerant flow communicationbetween the flash tank 70 and refrigerant line 6 of the primaryrefrigerant circuit at a point downstream with respect to refrigerantflow of the outlet of the evaporator 50 and upstream with respect torefrigerant flow of the suction inlet to the compression device 20.Refrigerant vapor collecting in the portion of the flash tank 70 abovethe liquid level therein passes from the flash tank 70 throughrefrigerant line 14 to enter the primary refrigerant circuit to returnto the compression device 20. A flow control valve 85 is interdisposedin refrigerant line 14 to restrict the flow of refrigerant vapor throughrefrigerant line 14 as necessary to maintain the separation chamber 72of the flash tank 70 at a refrigerant pressure higher than suctionpressure. In an embodiment, the flow control valve 85 comprises asolenoid valve having a first open position and a second closedposition, such as for example, but not limited to, a pulse widthmodulated solenoid valve. In an embodiment, the flow control valve 85may comprise an electronic expansion valve.

Liquid refrigerant collecting in the lower portion of the flash tankeconomizer 70 passes therefrom through refrigerant line 4 and traversesthe primary refrigerant circuit expansion valve 55, which may be anelectronic expansion valve or a conventional thermostatic expansionvalve, disposed in refrigerant line 4 upstream with respect torefrigerant flow of the evaporator 50. As this liquid refrigeranttraverses the first expansion device 55, it expands to a lower pressureand temperature before entering the evaporator 50. As the liquidrefrigerant passes through the evaporator 50, the liquid refrigerantpasses in heat exchange relationship with a heating medium whereby theliquid refrigerant is vaporized and typically superheated and theheating medium is cooled. In an embodiment, the evaporator 50constitutes a finned tube coil heat exchanger 52, such as a fin andround tube heat exchanger or a fin and flat, mini-channel tube heatexchanger. The heating fluid passed in heat exchange relationship withthe refrigerant in the evaporator 50 may be air drawn by an associatedfan(s) 54 from a climate controlled environment, such as aperishable/frozen cargo storage zone associated with a transportrefrigeration unit, or a food display or storage area of a commercialestablishment, or a building comfort zone associated with an airconditioning system, to be cooled, and generally also dehumidified, andthence returned to a climate controlled environment. The low pressurerefrigerant vapor leaving the evaporator 50 returns through refrigerantline 6 to the suction inlet of the compression device 20.

As in conventional practice, the primary expansion valve 55 meters therefrigerant flow through the refrigerant line 4 to maintain a desiredlevel of superheat in the refrigerant vapor leaving the evaporator 50and passing through refrigerant line 6 to ensure that no liquid ispresent in the refrigerant leaving the evaporator. As noted before, theprimary expansion valve 55 may be an electronic expansion valve, inwhich case the expansion valve 55 meters refrigerant flow in response toa signal from a controller 100 to maintain a desired suction temperatureor suction pressure in refrigerant line 6 on the suction side of thecompression device 20. The primary expansion device 55 may also be athermostatic expansion valve in which case the expansion valve 55 metersrefrigerant flow in response to a signal indicative of the refrigeranttemperature or pressure sensed by the sensing device, which may be aconventional temperature sensing element, such as a bulb or thermocouplemounted to the refrigerant line 6 in the vicinity of the evaporatoroutlet.

In the exemplary embodiment of the refrigerant vapor compression system10 depicted in FIG. 1, operation of the refrigerant vapor compressionsystem is controlled by a control system that includes a controller 100operatively associated with the flow control valve 85 interdisposed inrefrigerant line 14 and the economizer circuit expansion device 65interdisposed in refrigerant line 12. The controller 100 also maycontrol operation of the electronic expansion valves 55 and 65, thecompression device 20, and the fans 44 and 54. As in conventionalpractice, in addition to monitoring ambient conditions, the controller100 may also monitors various operating parameters by means of varioussensors operatively associated with the controller 100 and disposed atselected locations throughout the system. For example, in the exemplaryembodiments depicted in FIG. 1, a pressure sensor 102 may be disposed inoperative association with the flash tank 70 to sense the pressurewithin the flash tank 70, a temperature sensor 103 and a pressure sensor104 may be provided to sense the refrigerant suction temperature andpressure, respectively, and a temperature sensor 105 and a pressuresensor 106 may be provided to sense refrigerant discharge temperatureand pressure, respectively. The pressure sensors 102, 104, 106 may beconventional pressure sensors, such as for example, pressuretransducers, and the temperature sensors 103 and 105 may be conventionaltemperature sensors, such as for example, thermocouples or thermistors.

The refrigerant vapor compression system of the invention isparticularly adapted for operation in a transcritical cycle with a lowercritical point refrigerant such as carbon dioxide, but may also beoperated in a subcritical cycle with a conventional higher criticalpoint refrigerant. When the refrigerant vapor compression system 10 isoperating in an economized mode, the controller 100 controls theeconomizer circuit expansion device 65 to meter the flow of refrigerantvapor from refrigerant line 4 through the economizer circuit refrigerantline 12 in response to system operating conditions and capacityrequirements. When the system is operating in a non-economized mode, thecontroller 100 closes the economizer circuit expansion valve 65 so thatall of the refrigerant passing from the gas cooler 40 throughrefrigerant line 4 passes through the secondary expansion device 75 andthence into the flash tank 70. In either the economized ornon-economized modes, the controller 100 controls the primary expansionvalve 55 to meter the correct amount of refrigerant liquid out of theflash tank 70 in response to the sensed system operating parameters, forexample compressor discharge temperature, to match the refrigerantcharge demand of the system.

Additionally, the controller 100 controls positioning of the flowcontrol valve 85 interdisposed in refrigerant line 14 to restrict theflow of refrigerant vapor from the flash tank 70 in response to thesensed pressure within the separation chamber flash tank 70 so as tomaintain a desired subcritical flash tank pressure. As the ratio ofrefrigerant liquid to refrigerant vapor present in the flash tank willdepend upon the subcritical pressure level within the separationchamber, the flash tank pressure may be controlled through positioningof the flow control valve 85 so as to produce a selected refrigerantquality upon expansion. If the flow control valve 85 is continuouslyclosed, the pressure within the flash tank with rise to an upper limitof the gas cooler pressure. If the flow control valve 85 is continuouslyopen, the pressure within the flash tank 70 will fall to a lowerpressure, but above the suction pressure. The actual pressuredifferential between the pressure within flash tank and suction pressurewhen the flow control valve is fully open will be governed by the sizeof the orifice in the particular flow control valve used. The controlleddischarge of refrigerant vapor from the flash tank 70 throughrefrigerant line 14 to suction pressure is essential for maintaining alow pressure within the flash tank. Therefore, the controller 100 mayalso continuously cycle the flow control valve 85 between its open andclosed positions to selectively control the flash tank pressure. Thismanipulation of the primary expansion valve 55 and the flow controlvalve 85 provides the controller 100 with the ability to effectivelymanage refrigerant charge over a wide range of operating conditions evenwhen the refrigerant vapor compression system 10 is operating in atranscritical mode. Additionally, separating the refrigerant into itsliquid and vapor phases in the flash tank 70 and sending only the liquidrefrigerant through the evaporator, while diverting the vaporrefrigerant to a point downstream of the evaporator, improves theeffectiveness of heat exchange in the evaporator.

A comparison of the pressure to enthalpy relationship presented in FIG.2, which represents a characteristic pressure to enthalpy relationshipfor the refrigerant vapor compression system 10 of FIG. 1, to either ofthe pressure to enthalpy relationships representative of conventionalrefrigerant vapor compression systems presented in FIG. 3 or FIG. 4,illustrates the capacity improvement associated with the refrigerantvapor compression system of the invention. FIG. 3 presents acharacteristic pressure to enthalpy relationship for a conventionalprior art transcritical refrigerant vapor compression having a singlerefrigerant-to-refrigerant heat exchanger economizer. FIG. 4 presents acharacteristic pressure to enthalpy relationship for a conventionalprior art transcritical refrigerant vapor compression having a singleflash tank economizer. In each of FIGS. 2-4, AB represents the gas heatrejection process within gas cooler 40 and DE represents the gas heatabsorption process within the evaporator 50. In FIG. 2, KG representsthe process within the refrigerant-to-refrigerant heat exchangereconomizer circuit and MN represents the process within the flashtank-to-suction evaporator bypass circuit. In FIG. 3, KG represents theprocess within the refrigerant-to-refrigerant heat exchanger economizercircuit. In FIG. 4, JL represents the process within a flash tankeconomizer circuit. The evaporator line DE in FIG. 1 is longer than therespective evaporator lines associated with either of the prior artsingle economizer systems, indicating the increased evaporatoreffectiveness associated with the refrigerant vapor compression systemof the invention.

Those skilled in the art will recognize that many variations may be madeto the particular exemplary embodiments described herein. While thepresent invention has been particularly shown and described withreference to the exemplary embodiment as illustrated in the drawings, itwill be understood by one skilled in the art that various changes indetail may be effected therein without departing from the spirit andscope of the invention as defined by the claims.

1. A refrigerant vapor compression system comprising: a primaryrefrigerant circuit including a refrigerant compression device, arefrigerant cooling heat exchanger for passing refrigerant received fromsaid compression device at a high pressure in heat exchange relationshipwith a cooling medium, a refrigerant heating heat exchanger for passingrefrigerant at a low pressure refrigerant in heat exchange relationshipwith a heating medium, and a primary expansion device interdisposed inthe primary refrigerant circuit downstream of said refrigerant coolingheat exchanger and upstream of said refrigerant heating heat exchanger;a refrigerant-to-refrigerant heat exchanger economizer having a firstrefrigerant pass disposed in the primary refrigerant circuit downstreamof said refrigerant cooling heat exchanger and upstream of said primaryexpansion device and a second bypass disposed in an economizer circuitrefrigerant line; a flash tank disposed in the primary refrigerantcircuit downstream of the first refrigerant pass of saidrefrigerant-to-refrigerant exchanger and upstream of said primaryexpansion device, said flash tank defining a separation chamber whereinrefrigerant in a liquid state collects in a lower portion of saidseparation chamber and refrigerant in a vapor state in a portion of saidseparation chamber above the liquid refrigerant; a secondary expansiondevice disposed in the primary refrigerant circuit in operativeassociation with and upstream with of said flash tank; an refrigerantvapor bypass line establishing refrigerant flow communication between anupper portion of said separation chamber of said flask tank and asuction pressure portion of said primary refrigerant circuit downstreamof said refrigerant heat absorption heat exchanger; and a bypass flowcontrol valve interdisposed in said evaporator bypass line, said bypassflow control valve having a first open position whereat refrigerantvapor may pass through said evaporator bypass line and a second closedposition whereat refrigerant vapor is blocked from passing through saidevaporator bypass line.
 2. A refrigerant vapor compression system asrecited in claim 1 wherein said flow control valve comprises a solenoidvalve having a first open position and a second closed position.
 3. Arefrigerant vapor compression system as recited in claim 1 wherein saidflow control valve comprises a pulse width modulated solenoid valve. 4.A refrigerant vapor compression system as recited in claim 1 whereinsaid flow control valve comprises an electronic expansion valve.
 5. Arefrigerant vapor compression system as recited in claim 1 wherein saidprimary expansion device comprises an electronic expansion valve.
 6. Arefrigerant vapor compression system as recited in claim 1 wherein saidprimary expansion device comprises a thermostatic expansion valve.
 7. Arefrigerant vapor compression system as recited in claim 1 wherein saidsecondary expansion device comprises an electronic expansion valve.
 8. Arefrigerant vapor compression system as recited in claim 1 wherein saidsecondary expansion device comprises a fixed orifice expansion device.9. A refrigerant vapor compression system as recited in claim 1 whereinsaid economizer circuit refrigerant line extends in refrigerant flowcommunication from said primary refrigerant circuit to an intermediatepressure stage of said compression device.
 10. A refrigerant vaporcompression system as recited in claim 9 further comprising aneconomizer circuit expansion device interdisposed in said economizercircuit refrigerant line upstream with respect to refrigerant flow ofthe second refrigerant pass of said refrigerant-to-refrigerant heatexchanger economizer.
 11. A refrigerant vapor compression system asrecited in claim 10 wherein said economizer circuit expansion devicecomprises an electronic expansion valve.
 12. A refrigerant vaporcompression system as recited in claim 10 wherein said economizercircuit expansion device comprises a thermostatic expansion valve.
 13. Arefrigerant vapor compression system as recited in claim 1 wherein saidcompression device comprises a single compressor having at least twocompression stages.
 14. A refrigerant vapor compression system asrecited in claim 1 wherein said compression device comprises at leasttwo compressors disposed in the refrigerant circuit in a seriesrelationship with respect to refrigerant flow.
 15. A refrigerant vaporcompression system as recited in claim 1 wherein said compression devicecomprises a scroll compressor.
 16. A refrigerant vapor compressionsystem as recited in claim 1 wherein said compression device comprises areciprocating compressor.
 17. A refrigerant vapor compression system asrecited in claim 1 wherein said compression device comprises a screwcompressor.
 18. A refrigerant vapor compression system as recited inclaim 1 wherein said system is incorporated in a transport refrigerationsystem for conditioning a temperature controlled cargo storage region.19. A refrigerant vapor compression system as recited in claim 18wherein said system operates in a transcritical cycle.
 20. A refrigerantvapor compression system as recited in claim 19 wherein the refrigerantcomprises carbon dioxide.